comprising

A transitional phrase used in claims that means including, containing, characterized by. The term is inclusive or open-ended and does not exclude additional, unrecited elements or steps. MPEP 2111.03.

The word "comprising" in a claim renders the claim open, which means that additional elements could be added to the accused infringing device without avoiding infringement. For example, if a claim stated that a chair had three legs and employed the word "comprising," a chair having four legs would infringe. At the other end of the spectrum is a closed claim, which uses the expression "consisting of." This means that in a product alleged to infringe, if meaningful additional elements are added to the features recited in a claim there is no infringement. These types of claims frequently appear in the chemical and metallurgical arts. There is an intermediate scope of claim that uses the expression "consisting essentially of" and means that some additional components may be employed in the allegedly infringing product without avoiding infringement.

(used when defining the scope of a claim)

A transitional phrase that is synonymous with (means the same thing as) "including," "containing" or "characterized by;" is inclusive or open-ended and does not exclude additional, unrecited elements or method steps. Comprising is a term of art used in claim language which means that the named elements are essential in describing the invention.

-- see MPEP 2111.03 for more